That is a great question "How did I discard the front ABS pick up rings"

At the moment I have not scientifically discarded them, more I have assumed based on the deliberate speed error. (Assumptions and diagnosis do not go hand in hand, I should know that)

At 70 mph (Speedometer reading) the front ABS sensors are reading approximately 67-68 mph which is the kind of deliberate error we see with speedometers (Where as the rears are reporting an uncharacteristic erroneous lower speed)

This is an assumption and so I need to apply a math channel based on ABS sensor frequency and wheel circumference on my next test to confirm which Pick up ring is incorrect (Front or rear)

I guess I could also use a chaperone following in a vehicle behind the Subaru to indicate when they are travelling at 70 mph. (Not so scientific but slightly quicker)

Steve Smith wrote:...At the moment I have not scientifically discarded them, more I have assumed based on the deliberate speed error. (Assumptions and diagnosis do not go hand in hand, I should know that)

At 70 mph (Speedometer reading) the front ABS sensors are reading approximately 67-68 mph which is the kind of deliberate error we see with speedometers (Where as the rears are reporting an uncharacteristic erroneous lower speed)...

Not a scientific method I agree, but if I had to guess, based on the speedometer 70mph, the front wheels 68mph are the ones giving the right speed. The error could be more than that, I agree, and the rear ones could be the correct ones, but less likely I guess.

But, more than that, my question is if that car has some type of speed sensor in the gearbox? If not, the speedometer value must be from one of the wheels abs sensor, right? I have seen front ones, but it could be rear ones in some cars, don't know...In that case, the speedometer value is not a valid way to validate it, and we have the same probability to have a wrong speed reported from the front, or from the rear wheels.

This is an assumption and so I need to apply a math channel based on ABS sensor frequency and wheel circumference on my next test to confirm which Pick up ring is incorrect (Front or rear)

front wheel speeds at 68 mph, rear wheel speeds at 62 mph!

Indeed the Math Channel based on WSS Frequency & Wheel Circumference would be belt and braces for accuracy, however your discrepancy is not small, greater than 10%. A GPS device (tom tom) I expect would suffice as a quick, reasonably accurate cross check of serial data ?

I guess I could also use a chaperone following in a vehicle behind the Subaru to indicate when they are travelling at 70 mph. (Not so scientific but slightly quicker)

Car speedometers tend to over report speed by a small percentage, rarely correlate accurately with Serial data. That idea may be quick, but quick to only muddy the waters and possibly lead you up the wrong garden path.

Hi Steve and AllIt's odd really as I could swear one of the UK trade magazines had an almost identical issue in a technical feature, in their instance only 1 Wheel bearing/hub (can't remember which - but it contained the ring for the abs, not sure if the sensor was integrated or external), but the overall result was that the ABS was throwing up spurious "intermittent" errors, not sure if pump was being activated.

Turns out after that an aftermarket part, the wrong one, was fitted, with a different pulse count ...

It might be that one axle has been recently replaced with the wrong part ...

So, with no access to previous owner and their service/repair history, you goal will be to decide which axle is right and which is wrong ... (right probably being the pair not recently replace, but the previous owner might have replaced all 4 at 2 different garages before selling the problem on!) ... if you can't find a Subaru dealer with the technical data, then maybe a shout out to fellow Pico Forum members with a similar car, to do a simple test ... possible logging some OBD data parameters that you can use to confirm the pulse count?

The noise however that starts 30ms prior to the ABS pump intervention is concerning for me, not part of this issue I suspect, but maybe an early indicator of some other pending issue (pump or ECU or relay or ...)?

I did need to check the date of the original post, as thought this might have been related to the original article and just new activity! Sadly can't find the article, as it was not really technical, no mention of the diagnosis otherwise I'd have scanned it. Maybe some else remembers reading it or can find it on google, where I failed!

Before I do, thank you for the GPS road speed verification tip (sometimes we over complicate things for sure) What was I thinking, Chaperone in another vehicle!

Richard, you are right on the money with the bearings!

I must say a big thank you again to Kevin at Ives Garage who called me when a Subaru Legacy turned up for an MOT

I was over there faster than you can say "Subaru" to carry out the optical sensor test (ABS Tooth count)

Using the optical sensor kit with a “pick-up” placed at a strategic point about the road wheel, a trigger could be generated for PicoScope to denote one revolution of the road wheel.

Donor Optical Test

The pulses from each ABS pick-up ring were counted and guess what? 48 teeth Front and Rear! At Last a real breakthrough

Donor Subaru front and rear tooth count

The luxury of a donor vehicle for measurements is a slice of heaven.

This then leads me full circle back to my colleague who mentioned something I chose to omit at my peril! (Customer interview mentioned a replacement wheel bearing to the right rear only)

When the car was purchased one of the rear wheel bearing was exceptionally noisy and so 4 x new wheel bearing/hub assemblies were purchased from a famous internet auction site!

Only the two rear bearings were replaced (not the front)

I can see you are ahead of me now.

The original rear wheel bearings have been discarded but I do have the new front wheel bearings purchased with the rears on my desk awaiting inspection. (I have a feeling they will have 44 teeth)

If this is the case, if all 4 wheel bearings had been installed together, no ABS fault would have occurred and only a speedo error would have followed. (I will confirm the tooth count ASAP)

Thank you now goes to Charles Figgins at Bilstein Group for inspecting a stock wheel bearing using a beautiful simple tool used to determine wheel bearing orientation when supplied with an integrated ABS Pick Up ring.

Tooth count-Bearing orientation

Charles was kind enough to mark the circumference of the hub assembly at strategic points to allow him to use the bearing tool to reveal and count the number of teeth/poles about the ABS pick up ring. 96 teeth/poles / 2 = 48

Hi SteveWell done, pleased you got the scientific evidence to support the theory

Sadly, I fear this type of issue is going to be more prevelent in the future, with cheap or alternative supply parts being sold as covering a wider vehicle parc than they really do and many owners not fessing up to the full facts when the car comes in with problems, often quite obtuse in terms of cause and effect ... I doubt wheel speed sensors will be the only component, but being integrated into the wheel bearing and replaced due to bearing noise or excessive play will often leave the owner unaware and disjoined from the cause and effect ... so often editing in their own mind the history and facts ...

A broader awareness on our part, as repairers and diagnosticians, of these types of mistakes will undoubtedly save us time and give us confidence when faced with similar situations, of course backed up by correct test procedures. Without these sort of case studies you'd be left doubting your initial findings, and possibly continue testing for other explanations, and getting a very sore head ...

We recently had a fuel filter on a V8 (client changed them himself) from a reputable aftermarket Brand, but bought on said global auction site (so based on our final findings doubt they were genuine), that randomly resulted in no effective flow/pressure relief (ie excessive levels of) pre-fuel rail, and after fixing other things finally caught the syptoms of failed pressure regulator return valve ... an item costing £400+ (verse the real culprit a £20 filter) but then got thinking about the filters and did more tests ... eureka moment ...

So a V8 with twin fuel pumps, twin filter, twin returns (one for each bank of 4), one bank was ok, the other randomly not, was giving all sorts of rough running, misfire ... and of course, having been told the filters had been replaced (reportedly to cure the problem), we never consider the filters ... until ruled everything else out ... and boy did we burn some time ... found 3 other meaningful issues (probably the one the filters were changed for!), but not to fix the new fault ... what A PITA, but live and learn.

Moral of the story, don't trust the work someone else has done, or the parts they have fitted. Start from scratch, and don't rule anything out unless you can prove it yourself !!

FioranoCars wrote:The noise however that starts 30ms prior to the ABS pump intervention is concerning for me, not part of this issue I suspect, but maybe an early indicator of some other pending issue (pump or ECU or relay or ...)?

Be nice to know your thoughts, as now clearly not part of the problem or it's solution, as hopefully the ABS will not trigger, and the noise hence will be gone ... but any idea what was causing it? - Amps in rush to the pump? not sure about that ... as the pump will be running regularly to accumulate pressure ...- Part of the ABS solenoid unit (valve that triggers the ABS reaction?)?

and many owners not fessing up to the full facts when the car comes in with problems,

Not only the owners, the previous unsuccessful repairers too.

Steve."New Feature Request" is normally Richard territory and he does it far more eloquently than I ever could hope too. However, dare I jump on his bandwagon and request one of my own ? It is Christmas (soon)

Can the Picoscope, perhaps with the NVH and / or the long back probe needles be adapted to function as an "Intrusive" or a "Very Intrusive" Customer Lie detector? If Jeremy Kyle can do it then .........